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Last weekend in Downing Street, David Cameron looked over the edge. The intelligence reaching Number 10 from the front line in Scotland was that Alex Salmond was building a momentum for independence that might be unstoppable.

What followed was a chaotic fightback by the No campaign, featuring Gordon Brown, an emotional Mr Cameron, oil barons and bank chiefs, all seeking to stop Mr Salmond’s surge.

“We had to act fast – there was a real risk Salmond’s bandwagon would become unstoppable,” admitted one No campaigner.

By Friday, there was some breathing space for Mr Cameron and the beleaguered Better Together campaign. An ICM poll for the Guardian put the No side ahead by 51 to 49 – suggesting that Mr Salmond’s momentum had stalled.

However, an average of six opinion polls last week gave Yes 48.7, confirming that the campaign for Scottish independence is within touching distance of victory. The outlook for the 307-year-old UK remains critical.

The mood in Downing Street last weekend was bleak, as a YouGov poll gave the Yes side the lead for the first time. Douglas Alexander, Labour’s campaign chief in Scotland, and Danny Alexander, the Liberal Democrat Treasury minister, knew they needed to reboot their campaign urgently.

In the Better Together headquarters there was a fear that Mr Salmond would ride a tide of euphoria this week, turning the march towards separation into “a carnival”; the independence party would be so good that no one would bother to worry about the bill until later.

The first stage of the fightback was to deploy Mr Brown on Monday to outline a breakneck timetable for delivering new powers over tax and welfare to the Scottish parliament, if Scotland voted No next Thursday.

The co-ordination of Mr Brown’s statement at the Loanhead miners’ welfare club was shambolic, but at least it gave No campaigners something positive to say to Labour voters who were considering voting Yes.

As Mr Brown put it on Friday night at a Labour rally in Glasgow: “This is the week that locked in faster, better, safer change for a stronger Scotland in the United Kingdom.” It will become a mantra in the closing days of the campaign: you do not have to vote Yes to get more powers for Scotland.

Meanwhile, Mr Cameron began turning the screws on reluctant business leaders to speak out against independence, issuing a “call to arms” at a Downing Street reception. Andrew Dunlop, his Scottish adviser, frantically bashed the phones.

The result was a trickle and then a torrent of chief executives warning that Scots would face higher food bills, higher taxes and the flight of their biggest banks – or at least their registered headquarters – if they were to vote Yes next week.

Mr Salmond and his team considered their response. The decision was not to engage in a detailed debate on the substance of the individual claims – rather to dismiss the business onslaught as “scaremongering” orchestrated by shadowy figures at Westminster and fanned by a hostile BBC.

In depth

Those in the Yes campaign believe that by relentlessly focusing on the “positive message” they can drown out the chorus of business criticism. Mr Salmond told a rally on Thursday that the Scots were “finding their voice” and were “on the cusp” of making history.

Mr Cameron and Ed Miliband’s decision to cancel their weekly question time bout at Westminster for a crisis visit to Scotland heightened the impression of panic. But the polls suggest that this week’s flurry of activity has at least stabilised a No campaign that Mr Salmond claimed was “in terminal decline”.

Blair Jenkins, chief executive of the Yes campaign, said the closeness of the polls proved that “David Cameron’s co-ordination from Downing Street of big business to talk down Scotland has been met with a giant raspberry, particularly among Labour voters”.

But with as many as 17 per cent of voters undecided, according to ICM, both sides know that the outcome of this remarkable two-year referendum campaign may not be known before dawn starts to break over Scotland next Friday.